William Hague and Michael Portillo yesterday finally slapped down their shadow cabinet colleague Oliver Letwin, who embarrassed the leadership last week by suggesting that a Tory government could slash taxes by £20bn - more than double the amount pledged.

In television and radio appearances, the two most prominent Tories said that a series of multi-billion pound tax cuts mentioned by the shadow chief secretary to the Treasury were wrong. "The figures are not right," Mr Portillo said of the claim by his deputy that the Tories could cut taxes by £20bn within five years.

Mr Letwin came close to derailing the Tory campaign last week when he said that he was "190%" confident that the Tories could cut taxes beyond the £8bn pledged by 2004. In an interview with the Financial Times, which the Tories initially tried to dismiss as inaccurate, Mr Letwin said that tax cuts could increase from £4bn in the first year to £20bn by the fifth.

His entire projections were dismissed by Mr Portillo on BBC1's On the Record yesterday. The shadow chancellor, who last week claimed that no Tory had mentioned the £20bn figure, said: "The figures are not right. I have made it perfectly clear that in the first budget I am only committed to £2.2bn worth of tax cuts and that is to produce the reduction in the tax on fuel ... At the end of my second year, I will have produced £8bn of tax cuts."

Within minutes, Mr Hague popped up on BBC Radio 4 to reject Mr Letwin's claim that public spending could be cut from 40% of GDP to 35%. Informed this would lead to a £50bn cut in public spending, he told The World This Weekend: "Clearly that is not our policy. We have no target of a proportion of GDP."

Mr Hague and Mr Portillo both said they hoped to introduce tax cuts of more than £8bn when the current public spending round comes to an end in 2004. However, they stopped short of repeating Mr Letwin's mistake and declined to put a figure on how much the cuts would be after 2004.

Labour seized on their remarks. Andrew Smith, chief secretary to the treasury, said: "Tory economic policy has descended into chaos as Michael Portillo and William Hague have failed to deny the secret Tory plan to cut £20bn from Britain's public services."

However, the Tories attempted to step up the pressure on Labour by seizing on the refusal of Gordon Brown, the chancellor, to say whether he would increase the ceiling on national insurance contributions.

Mr Portillo pointed out that Mr Brown had promised before the last election to maintain the ceiling on NI contributions, only to increase it well above the rate of inflation.

Mr Hague insisted yesterday that the Conservatives could win the election, despite two weekend polls showing Labour with a 19 point lead. "We are certainly winning the campaign. Millions of people in this country are undecided about how to vote," he said.